Liquid crystal displays are broadly grouped under transmissive display devices, reflective display devices, and semi-transmissive display devices. The transmissive display devices adopt, as a light source, light from a lighting unit (back light) arranged at the rear of a liquid crystal panel. The reflective display devices adopt, as a light source, surrounding light. The semi-transmissive display devices have properties both as transmissive ones and reflective ones.
Of these, the transmissive display devices create a display by allowing back light irradiated from the rear side of the liquid crystal panel to pass through the liquid crystal panel and a color filter and thereby have advantages such that a display can be indicated at a high contrast ratio without being affected by the environment of use. They are generally used in large-sized electronic appliances which require satisfactory brightness, such as television sets and personal computer monitors. However, they are somewhat unsuitable for small appliances such as cellular phones, because they require power for the back light.
In contrast, the reflective display devices create a display by allowing natural light or artificial light to reflect in a liquid crystal panel and allowing the reflected light to pass through the liquid crystal panel and a color filter. They do not need the back light and are thereby generally used typically in electronic desk calculators and clocks (or watches). The reflective display devices, however, are disadvantageous in that the brightness and contrast ratio of the display are largely affected by the surrounding environment upon use and, in particular, it is difficult to see the display in dark surroundings.
In contrast to these, the semi-transmissive display devices create a display both by a transmissive mode and a reflective mode according to the surroundings upon use. Typically, they use a reflective electrode to save electrical power consumption in daytime, but they allow the back light to illuminate to create a display according to necessity in a room or at nighttime. They are therefore advantageous in that they can save the electrical power consumption without being limited by the surrounding environment and can create a display at a high contrast ratio. The semi-transmissive display devices are therefore optimally used in mobile devices and are generally used particularly typically in colorized cellular phones.
The structure (configuration) and operating principles of a representative semi-transmissive liquid crystal display device will be illustrated with reference to FIG. 1 and FIG. 2. FIG. 1 and FIG. 2 correspond to FIG. 1 and FIG. 2 disclosed in Patent Document 3 mentioned below.
With reference to FIG. 1, a semi-transmissive liquid crystal display device 11 includes a thin film transistor (hereinafter referred to as TFT) substrate 21; a counter substrate 15 arranged so as to face the TFT substrate 21; and a liquid crystal layer 23 arranged between the TFT substrate 21 and the counter substrate 15 and functions as an optical modulation layer. The counter substrate 15 includes a color filter 17 including a black matrix 16; and a transparent common electrode 13 is arranged on the color filter 17. Independently, the TFT substrate 21 includes pixel electrodes 19, switching elements T, and a wiring part including scanning lines and signal lines. In the wiring part, two or more gate wirings 5 and two or more data wirings 7 are arranged perpendicularly to each other. TFTs as switching elements (indicated as “T” in the figure) are arranged in matrix at intersecting portions where the gate wirings 5 and the data wirings 7 intersect each other.
As is illustrated in detail in FIG. 2, pixel areas P of the pixel electrodes 19 each include a transmissive area A and a reflective area C. A transparent electrode (pixel electrode) 19a is present both in the transmissive area A and the reflective area C; and a reflective electrode 19b is present in the reflective area C. A barrier metal layer 51 is arranged between the transparent electrode 19a and the reflective electrode 19b. The barrier metal layer 51 is made from a high-melting-point metal such as Mo, Cr, Ti, or W. This layer will be illustrated in detail later.
The operating principles of transmissive mode and reflective mode of the semi-transmissive liquid crystal display device 11 shown in FIG. 1 will be illustrated with reference to FIG. 2.
Initially, the operating principles of transmissive mode will be illustrated.
In the transmissive mode, light F from a back light 41 arranged below the TFT substrate 21 is used as a light source. The light emitted from the back light 41 passes through the transparent electrode 19a and the transmissive area A and enters the liquid crystal layer 23. An electric field formed between the transparent electrode 19a and the common electrode 13 controls the alignment direction of liquid crystal molecules in the liquid crystal layer 23; whereby the incident light F emitted from the back light 41 and passing through the liquid crystal layer 23 is modulated. This controls the quantity of light passing through the counter substrate 15 to create a display of image.
In contrast, external natural light or artificial light B is used as a light source in the reflective mode. The light B coming into the counter substrate 15 is reflected by the reflective electrode 19b. An electric field formed between the reflective electrode 19b and the common electrode 13 controls the alignment direction of liquid crystal molecules in the liquid crystal layer 23, whereby the light B passing through the liquid crystal layer 23 is modulated. This controls the quantity of light passing through the counter substrate 15 to create a display of image.
The pixel electrode 19 includes the transparent electrode 19a and the reflective electrode 19b. Of these, the transparent electrode 19a is formed representatively from an electroconductive oxide film typically of an indium tin oxide (ITO) or indium zinc oxide (IZO). The indium tin oxide (ITO) contains indium oxide (In O3) and about 10 percent by mass of tin oxide (Sn). The indium zinc oxide (IZO) contains indium oxide and about 10 percent by mass of zinc oxide.
The reflective electrode 19b is made from a metal material having a high reflectance represented by pure aluminum or an aluminum alloy such as Al—Nd (hereinafter these are synthetically referred to as “aluminum-based alloy”). Aluminum (Al) has also a low electric resistivity and is thereby very useful as a wiring material.
With reference to FIG. 2, the barrier metal layer 51 made from a high-melting-point metal such as molybdenum (Mo) is arranged between the aluminum-based alloy thin film constituting the reflective electrode 19b and the electroconductive oxide film, such as ITO or IZO, constituting the transparent electrode. This is because, if these films are directly connected to each other to form a reflective area, the contact resistance increases due typically to galvanic corrosion to thereby impair the display quality on the screen. Specifically, this problem occurs probably for the following reasons. Aluminum is very susceptible to oxidation. In addition, there is a large difference in electrode potential in an alkaline electrolyte (developer) between pure aluminum and the electroconductive oxide film; namely, pure aluminum has an electrode potential of −1.9 V, and in contrast, ITO has an electrode potential of −0.17 V. Accordingly, if the aluminum-based alloy thin film is directly connected to the electroconductive oxide film, an aluminum oxide insulating layer is formed at the interface between the two films by the action of oxygen formed or added during the deposition processes in the liquid crystal panel, and the aluminum oxide insulating layer may invite the above problem.
Based on this viewpoint, the techniques disclosed in Patent Document 1 to Patent Document 3, for example, have attempted to reduce the contact resistance by arranging a barrier metal layer made typically of No or chromium (Cr) between an aluminum-based alloy layer and a transparent pixel electrode (made typically of ITO).
Patent Document 1: Japanese Unexamined Patent Application Publication (JP-A) No. 2004-144826
Patent Document 2: Japanese Unexamined Patent Application Publication (JP-A) No. 2005-91477
Patent Document 3: Japanese Unexamined Patent Application Publication (JP-A) No. 2005-196172